If your vehicle is showing one headlight is completely out, here are the most likely causes and the parts you'll need to fix each one.
The filament inside the bulb has broken due to normal wear and tear, or the lighting element has simply reached the end of its lifespan.
Halogen bulbs burn extremely hot. If the glass was touched with bare fingers during a recent installation, the oils create a hot spot that causes the bulb to shatter or burn out very quickly.
The electrical connector plugging into the bulb can melt from excessive heat (often from aftermarket bulbs) or corrode from moisture intrusion, preventing power from reaching the bulb.
Many modern vehicles use separate fuses for the left and right headlights. A minor power surge or temporary short circuit can blow the fuse for just one side.
On vehicles with modern HID or integrated LED headlights, the internal ballast, wiring, or LED control board inside the housing can fail, often requiring replacement of the entire unit.